1. Field of the Invention
The present invention relates not only to a video signal processing apparatus for converting a still image video signal read out of a magnetic disk into a still digital image signal, which in turn is digitally processed, but also to a still video transmitter which displays on a CRT a reproduced signal read out of a video floppy for an electronic still camera and transmits a transmitted and reproduced video signal through a telephone line to an electric transceiver which has been used in the field of an information medium. For instance, the above-described video signal processing apparatus can be applied to a video signal processing apparatus of a still video transmitter in the so-called electric transmission system which transmits to a remote electrical/ transceiver through a telephone line a still image recorded on a floppy disk in an analog manner.
2. Related Background Art
In the case of transmitting a still video image through an electric transmission system, the transmission side must carry out video signal processing such as gamma correction depending upon various conditions for the transmission of the image to be transmitted, such as whether or not the distribution of lightness is wide, whether the whole image is rather light or dark, that is, whether the whole tone of the image is bright or dark, the sensitivity characteristic of a receiver and so on. In the case of gamma correction in the conventional electric transmission system, the video signal is subjected to A/D conversion and the digitally converted video signal is subjected to image processing (for instance, digital gamma correction) by a digital signal processing technique on the side of transmission. In the case of the A/D conversion, according to the conventional technique, the A/D conversion is carried out by the analog signal reproduced by one field when one frame is recorded by one field or by two fields when one frame is recorded by two fields. According to such A/D conversion, the analog video signal which becomes the base of the A/D conversion includes jitter due to fluctuation in rotation of a disk drive device. In the case of a moving image, adverse effects of jitter may be visually neglected, but in the case of a still image, when a vertical line is included it is distorted. That is, the received still image is adversely affected. Furthermore, in the case of the A/D conversion, the received still image is adversely affected by noise added during the process of the A/D conversion.
According to the above-mentioned technique, in order to eliminate the adverse effect on the received still image, even a small degree of jitter must be eliminated. For this purpose, variation in rotation of a disk drive device must be completely eliminated. However, there exists a problem that in order to satisfactorily eliminate the fluctuation in rotation of the disk drive device, the cost of the disk drive device is considerably increased. Furthermore there is not available an effective means for eliminating noise added in the process of the A/D conversion of the reproduced analog signal.
In the conventional news field, by utilizing telephone lines, transmitters and receivers, the video signal is transmitted to branch offices at remote places. In this case, a film exposed by a silver salt camera is developed, and the developed image is printed on a photographic paper or enlarged to form a silver salt photograph. The silver salt photo thus processed is wound around a drum of an electric transmitter while a photographic paper is wound around the drum of an electric receiver. Both the transmitter and receiver drums are rotated synchronized with each other and scanned so that the signal is transmitted for each line.
However, in the case of the above-mentioned electric transmission system, it takes a lot of time to confirm the actual image after the image was exposed and furthermore, an installation such as a dark room is required. Moreover, the transmitter is large in size so that it is difficult to move it. As a result, a finished print must be carried to the place where the electric transmitter is installed. In the field of information media which fights the clock, the loss in time is a very serious problem.